1. Field of the Invention
The present invention relates to a monitor television receiver for displaying a color video output.
2. Description of the Prior Art
In color monitor television receivers for business use particularly in broadcasting stations and so forth, it is necessary to control the current ratio (known generally as white balance) among electron guns of three primary colors in a cathode-ray tube so that a preset white color temperature can be maintained constant regardless of whether a video image is light (at a high luminance) or dark (at a low luminance).
Such white balance control is executed by adjusting a bias in each of drive circuits for three primary colors of red (R), green (G) and blue (B) when the R, G and B currents are in a non-controlled state as shown in FIG. 5A. First the biases in the R, G, B drive circuits are controlled to attain a coincidence of the low luminance levels as shown in FIG. 5B, and then the gains in the individual drive circuits are controlled to attain a coincidence of the drive level-to-luminance characteristic curves of the three primary colors as shown in FIG. 5C, thereby completing the adjustment.
However, any change caused in the bias by the control thereof affects the high luminance level, and also any change caused in the gain by the control thereof affects the low luminance level. Therefore, at the time of practical white balance control, it is necessary to repeat both the bias control and the gain control over again to gradually achieve proper bias and gain values, hence requiring an extremely complicated operation.
For the purpose of automating such operation, there has been developed an automatic setup system which measures the output light of a monitor television receiver by means of an optical sensor attached to the screen face of a cathode-ray tube (CRT), and controls a CRT drive circuit in such a manner that the color temperature of the measured output light becomes equal to a predetermined color temperature stored previously in a memory for example. The above development releases an operator from the complicated manual operation to perform the intricate procedure.
However, for execution of the white balance control according to such automatic setup system, an optical sensor needs to be attached to the CRT screen face. Therefore, when the monitor television receiver is installed at a high position or behind a control console for example in a monitor room of a broadcasting station, some difficulties may be occasionally existent in attaching the optical sensor to consequently raise a problem that the white balance control is not executable with facility. Furthermore, for achieving a substantially complete white balance on the basis of the actual value measured by the optical sensor, it is necessary that the optical sensor itself be excellent in spectral characteristic to eventually render the system extremely expensive.
In addition, it is impossible to completely eliminate the harmful influence of external light since the monitor output is measured on the CRT screen face, and an adjustment in any light place induces an increase in the error.
Besides the above, an accurate measurement of the color temperature of the monitor output light is impossible unless an expensive spectrum analyzer is employed and, even if a desired color temperature is specified by numerical values such as white of standard light D65 or white of coordinates (x, y)=(0.313, 0.319) on a CIE chromaticity diagram, it is difficult, depending on the performance of the optical sensor in general use, to attain control of the monitor output light in conformity with the numerical values or to control the white balance automatically with such values.
Due to the reason similar to the above, it is impossible to exactly measure the color temperature of the monitor output light after controlling the color temperature to a certain value or after controlling the white balance. Particularly in a multi-CRT display system or the like which displays one video image on a plurality of CRTs in a monitor room or an editing room of a broadcasting station, where a plurality of monitor television receivers need to be maintained in the same state under control, the color temperature cannot be specified by an absolute numerical value in each of the monitor television receivers to eventually raise a problem of extreme intricacy in the control manipulation.